Subquery testing

Since a subquery is just an expression that appears in the WHERE or HAVING clauses, the search conditions on subqueries may
look familiar.

They include:

Subquery comparison test
Compares the value of an expression to a single value produced by the subquery for each record in the table(s) in the main
query. Comparison tests use the operators (=, <>, <. <=, >, >=) provided with the subquery.

Quantified comparison test
Compares the value of an expression to each of the set of values produced by a subquery.

Subquery set membership test
Checks if the value of an expression matches one of the set of values produced by a subquery.